An On-Line UPS is usually operated in an on-line mode, a back-up mode and a bypass mode. When the On-Line UPS is operated in the On-Line mode, an ac output voltage is provided from a power source to a load through an inverter and a PFC converter. When the power source is abnormal and the UPS is operated in the back-up mode, the ac output voltage is provided from a battery to the load through the inverter and a dc/dc converter
For safety consideration, the output voltage of the On-Line UPS has a common middle line with the inputted power source by the incorporation of an isolated transformer at the output terminal. However, for an On-Line UPS without the isolated transformer, the PFC converter and the dc/dc converter must be able to output a positive and a negative dc voltages which are electrically connected to the middle line of the power source. The operation principles are described in detail in U.S. Pat. Nos. 6,661,678 and 5,654,591, etc.
Please refer to FIG. 1, which is a circuit diagram showing a converting device with PFC and dc/dc converting functions according to the prior art. The converting device 10 includes a PFC converter and a dc/dc converter. The PFC converter is composed of an ac input voltage vac, a relay J1, a filtering inductor L3, two rectifying diodes D9 and D10, two power switches S3 and S4, two diodes D11 and D12, and two filtering capacitors C1 and C2. The dc/dc converter is composed of a battery VBat, two power switches S1 and S2, a transformer Tr, eight diodes D1˜D8, two filtering inductors L1 and L2, and two filtering capacitors C1 and C2.
When the ac input voltage vac is provided normally, the relay J1 is switched to be electrically connected to the filtering inductor L3 so that the PFC converting function is achieved. When the ac input voltage vac is abnormal, the relay J1 is switched to be open to the filtering inductor L3 and the dc/dc converting function is then achieved by the dc/dc converter.
The positive bus voltage BUS+ and the negative bus voltage BUS− at the output terminal of the converting device 10 are inverted by the inverter into a low-frequency sinusoidal voltage and then outputted to the load. In the positive half cycle of the low-frequency sinusoidal voltage, the positive bus voltage BUS+ has an output power and the negative bus voltage BUS− does not. In the negative half cycle of the low-frequency sinusoidal voltage, the positive bus voltage BUS+ does not have an output power and the negative bus voltage BUS− does.
In the converting device shown in FIG. 1, the dc/dc converter and the PFC converter are two independent circuits. In the dc/dc converter, two circuits are adopted to be coupled to the secondary side of the transformer Tr to achieve a dual voltage output. The two circuits includes two secondary windings, two rectifying bridges composed of diodes D1˜D8, and two filtering inductors L1 and L2. With the two circuits, the converting device is able to operate in the positive and negative half cycles of the low-frequency sinusoidal voltage while the dc/dc converting function runs.
However, the traditional converting device 10 has too many electrical elements and each electrical element has a low utility rate. Consequently, the manufacturing cost for the traditional converting device 10 is quite high.